Imaging lens, camera and personal digital assistant

ABSTRACT

An imaging lens includes in order from an object side a first lens group including a positive refractive power and a second lens group having a negative refractive power, the first lens group and the second lens group including a widest air space therebetween, the first lens group including in order from the object side a negative first lens having a concave face on the object side, a positive second lens having a convex face on both sides, an aperture stop, a negative third lens having a concave face on the object side, a positive fourth lens having a convex face on an image side and a positive fifth lens having a convex face on the image side, and the second lens group including a negative meniscus sixth lens having a concave face on the object side.

PRIORITY CLAIM

The present application is based on and claims priority from JapanesePatent Application No. 2011-033612, filed on Feb. 18, 2011, thedisclosure of which is hereby incorporated by reference in its entirety.

BACKGROUND

1. Technical Field

The present invention relates to an imaging lens, a camera and apersonal digital assistant.

2. Description of the Related Art

There are a variety of functions and configurations used in a widespreaddigital camera. In recent years, a relatively large imaging elementhaving about 20-45 mm diagonal length is used in such a digital camera,and a high image quality and compact camera equipped with ahigh-performance single focus lens has greatly drawn user's attention.

Most of user's requests relate to portability, namely, compactness aswell as a high performance. A high performance requires at least a smallcollapse of a point image till a peripheral part of a field angle athigh contrast with small coma flare in an opened state of an aperturestop, non-generation of unnecessary coloring in a portion having a largebrightness difference with small chromatic aberration, and smalldistortion, in addition to a resolution corresponding to an imagingelement having 10-20 million pixels.

Moreover, compactness requires the entire length when normalizing with afocal length or the maximum image height shorter than that when using asmall imaging element because an actual focal length is increased due toa relatively large imaging element.

A large aperture at least less than F 2.8 is also required in view ofdifferentiation from a general compact camera equipped with a zoom lens.

Many users desire a certain level of wide-field angle, and it ispreferable for a half-field angle of an imaging lens to be 28° or more.A half-field angle of 28° corresponds to about 41 mm focal lengthconverted into a 35 mm film camera (so-called Leica size).

An image from an imaging lens is imaged by an imaging element (areasensor) in a digital camera. In view of the property of the area sensorhaving a color filter and a micro lens with respect to each pixel,various retrofocus type lenses in which a lens group having a negativerefractive power is arranged on the object side and a lens group havinga positive refractive power is arranged on the image side are proposedas a wide-angle single focus lens which moves the exit pupil positionaway from the image face and is suitable for entering a peripheral lightbeam onto the sensor at an angle close to the perpendicular.

However, such a retrofocus type increases an entire length (distancefrom most object-side face to image face) of a lens, so that a digitalcamera can not be easily downsized.

In recent years, owing to an improvement in and optimization of anon-chip micro lens and progress in an image process in an imagingelement having a relatively large diagonal length of about 20-45 mm, acertain level of oblique incidence of a peripheral light beam onto animaging face is allowed.

That is, a system which can sufficinently allow up to about 30° in anangle between a main light beam and an optical axis in the maximum imageheight can be created.

If such a system is used, a lens type suitable for downsizing can beselected without regard to the vertical incidence of the peripherallight beam.

A lens type which is more suitable for downsizing than the retrofocustype includes an approximate symmetric type and a telephoto type inwhich a lens group having a negative refractive power is arranged on animage side.

An imaging lens of this type is described in Patent Documents 1(Japanese Patent Application Publication No. S63-024213), 2 (JapanesePatent Application Publication No. H09-236746), 3 (Japanese PatentApplication Publication No. 2000-321490), 4 (Japanese Patent ApplicationPublication No. 2005-352060), 5 (Japanese Patent Application PublicationNo. 2009-216858), for example.

An imaging lens described in Patent Document 1 is a telephoto typehaving a four-group and four-lens configuration which is widely used ina compact film camera. It is small but it includes large distortion andastigmatism.

An imaging lens described in Patent Documents 2, 4 is small and has ahigh imaging performance, but it has 35° or more between the main lightbeam and the optical axis in the maximum image height. Because of this,it has a problem for use in a digital camera even if it is combined withthe above-described imaging element.

An imaging lens described in Patent Document 3 has a large aperture butit includes many lenses, creating rising costs. An imaging lensdescribed in Patent Document 5 has a large entire length compared with afocal length, so that it has a problem in downsizing.

An imaging lens described in Patent Documents 3-5 does not have arelatively large air space inside a lens system, and a large back focallength. For this reason, it is difficult to reduce the thickness of theimaging lens for housing even if a collapsed mechanism is used; thus, itis also difficult to downsize the imaging lens.

SUMMARY

The present invention has been made in view of the above circumstances,and an object of the present invention is to offer a small andhigh-performance imaging lens suitable for a digital camera with a wideangle in which a half-field angle is about 28-36°, a large aperture inwhich F-number is less than 2.8, a small number of lenses and aresolution corresponding to an imaging element having 10-20 millionpixels, and a camera and a personal digital assistant using the imaginglens.

In order to achieve the above object, one embodiment of the presentinvention provides an imaging lens including in order from an objectside a first lens group including a positive refractive power; and asecond lens group having a negative refractive power, the first lensgroup and the second lens group including a widest air spacetherebetween, the first lens group including in order from the objectside a negative first lens having a concave face on the object side, apositive second lens having a convex face on both sides, an aperturestop, a negative third lens having a concave face on the object side, apositive fourth lens having a convex face on an image side and apositive fifth lens having a convex face on the image side, and thesecond lens group including a negative meniscus sixth lens having aconcave face on the object side.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide further understandingof the invention, and are incorporated in and constitute a part of thisspecification. The drawings illustrate embodiments of the invention and,together with the specification, serve to explain the principle of theinvention.

FIG. 1 is a view illustrating a configuration of an imaging lensaccording to Embodiment 1.

FIG. 2 is a view illustrating a configuration of an imaging lensaccording to Embodiment 2.

FIG. 3 is a view illustrating a configuration of an imaging lensaccording to Embodiment 3.

FIG. 4 is a view illustrating a configuration of an imaging lensaccording to Embodiment 4.

FIG. 5 is a view illustrating a configuration of an imaging lensaccording to Embodiment 5.

FIG. 6 provides aberration curves when the imaging lens in Embodiment 1is focused on an infinite object.

FIG. 7 provides aberration curves when the imaging lens in Embodiment 1is focused on a close-range object by − 1/20 times.

FIG. 8 provides aberration curves when the imaging lens of Embodiment 2is focused on an infinite object.

FIG. 9 provides aberration curves when the imaging lens of Embodiment 2is focused on a close-range object by − 1/20 times.

FIG. 10 provides aberration curves when the imaging lens of Embodiment 3is focused on an infinite object.

FIG. 11 provides aberration curves when the imaging lens of Embodiment 3is focused on a close-range object by − 1/20 times.

FIG. 12 provides aberration curves when the imaging lens of Embodiment 4is focused on an infinite object.

FIG. 13 provides aberration curves when the imaging lens of Embodiment 4is focused on a close-range object by − 1/20 times.

FIG. 14 provides aberration curves when the imaging lens of Embodiment 5is focused on an infinite object.

FIG. 15 provides aberration curves when the imaging lens of Embodiment 5is focused on a close-range object by − 1/20 times.

FIG. 16 is a view describing a first embodiment of a personal digitalassistant.

FIG. 17 is a view describing a system of the personal digital assistantin FIG. 16.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an imaging lens, a camera and a personal digital assistantaccording to a first embodiment of the present invention will bedescribed. Firstly, the basic embodiment of these will be describedbefore describing specific embodiments including numerical values.

An imaging lens according to the first embodiment of the presentinvention includes in order from an object side a first lens grouphaving a positive refractive power and a second lens group having anegative refractive power. The imaging lens includes the widest airspace between the first lens group and the second lens group.

The first lens group includes in order from the object side a negativefirst lens having a concave face on the object side, a positive secondlens having a convex face on both sides, an aperture stop, a negativethird lens having a concave face on the object side, a positive fourthlens having a convex face on the image side, and a positive fifth lenshaving a convex face on the image side. The second lens group includes anegative meniscus sixth lens having a concave face on the object side.

The imaging lens includes six lenses as a whole.

As described above, the imaging lens includes a small number of lensessuch as six lenses as a whole, so that the size and costs of the imaginglens can be reduced.

In the imaging lens, it is preferable for a distance L from the mostobject-side face of the first lens group to an image face in a statefocused on an infinite object and a focal length f of the entire systemin a state focused on an infinite object to satisfy the followingcondition (1).

1.2<L/f<2.0   (1)

In the imaging lens, it is preferable for the focal length f of theentire system in a state focused on an infinite object and the maximumimage height Y′ to satisfy the following condition (2).

0.50<Y′/f<0.70   (2)

In the imaging lens, it is preferable for the distance L from the mostobject-side face of the first lens group to the image face in a statefocused on an infinite object and a distance D_(1G-2G) from the mostimage-side face of the first lens group to the most object-side face ofthe second lens group in a state focused on an infinite object tosatisfy the following condition (3).

0.15<D _(1G-2G)<0.50   (3)

When the above condition is satisfied, the field curvature can beeffectively reduced while the thickness of the imaging lens can befurther reduced for housing with a collapsed mechanism.

In the imaging lens, it is preferable for a focal length of the secondlens group f_(2G) and the focal length f of the entire system in a statefocused on an infinite object to satisfy the following condition (4).

−2.7<f _(2G) /f<−1.2   (4)

When the above condition is satisfied, the refractive power of thesecond lens group can be optimized and the imaging lens can be furtherdownsized.

In the imaging lens, it is preferable for a curvature radius R_(L3F) ofthe object-side face of the third lens and a distance D_(L2-L3) from theimage-side face of the second lens to the object-side face of the thirdlens to satisfy the following condition (5).

−2.5<R _(L3F) /D _(L2-L3)<−1.0   (5)

When the above condition is satisfied, the coma aberration can be wellcorrected.

In the imaging lens, it is preferable for a synthetic focal lengthf_(L1-L2) of the first and second lenses and the focal length f of theentire system in a state focused on an infinite object to satisfy thefollowing condition (6).

1.0<f _(L1-L2) /f<1.8   (6)

When the above condition is satisfied, the refractive power balance inthe first lens group can be optimized and the exit pupil position can besufficiently moved away from the image face.

In the imaging lens, it is preferable for the fifth lens to include asthe image-side face an aspheric shape in which a positive refractivepower weakens in its peripheral portion. In this case, it is preferablefor a distance L_(s-A) from the aperture stop to the aspheric surface ofthe fifth lens and a distance L_(s-I) from the aperture stop to theimage face to satisfy the following condition (7).

0.25<L _(S-A) /L _(S-I)<0.55   (7)

When the above condition is satisfied, the coma aberration andastigmatism can be well corrected.

In the imaging lens, the first lens can be only moved when focusing on aclose-range object.

By moving only the first lens group, the focusing mechanism can bedownsized and simplified while effectively controlling a change in animaging performance along with the focusing on a finite-range object.

A camera of this embodiment is a camera including the imaging lens as aphotographing optical system.

A personal digital assistant of this embodiment includes the imaginglens as a photographing optical system of a camera function portion.

Hereinafter, further description will be given.

The imaging lens includes a negative power on the most image side. Theentire configuration of the lens is made to be closer to a so-calledtelephoto type so as to reduce the entire length and the size of thelens.

A telephoto type imaging lens for use in a film camera generallyincludes a large incident angle of a peripheral light beam to an imageface, for example, 35-45°, which is not suitable for use in a digitalcamera. A small and high-performance imaging lens has a large number oflenses and a lot of aspheric surfaces, which result in an increase inthe costs.

On the contrary, an imaging lens having an incident angle of theperipheral light beam onto the image face of about 30° or below, whichis suitable for use in a digital camera, is not sufficient in terms ofboth downsizing and high performance. However, the above-described lensconfiguration can solve these problems.

That is, in order to provide the property of the telephoto type, thenegative first lens is arranged on the most object side relative to thenegative sixth lens (second lens group) arranged in the most image sidein view of the symmetric property, so that the coma aberration,chromatic aberration of magnification and distortion can be easilycorrected.

Moreover, with the concave face provided on the object-side face of thefirst lens, the coma aberration, chromatic aberration of magnificationand distortion can be corrected with a good balance.

Furthermore, the negative sixth lens is optimized as a field flattenerwith the meniscus shape having a convex face on the object side providedin the negative sixth lens, so that a high imaging performance can beachieved as a whole.

The four lenses from the second to fifth lenses include an inverseErnostar type as a whole, so that the spherical aberration and comaaberration can be preferably corrected even if a large aperture of lessthan F2.8 is used.

By providing the aperture stop between the second lens and the thirdlens which are arranged relatively on the object side in the lenssystem, the distance from the image face to the exit pupil iscontrolled, and the incident angle of the peripheral light beam onto theimage face can be prevented from being excessively increased.

More specifically, the object-side face of the third lens and theimage-side face of the fifth lens include an approximate concentricshape to the aperture stop to control the generation of the comaaberration while moving the exit pupil position away from the image faceby the positive refractive power of the fourth and fifth lenses.

The first lens group including the first to fifth lenses and the secondlens group including the sixth lens are arranged to have a wide airspace therebetween. This makes it possible to clearly separate into therole of the first lens group as the imaging group and the role of thesecond lens group having both of a rear convertor function and a fieldflattener function appropriately.

With this configuration, large aberration exchange beyond necessity isnot performed between the first lens group and the second lens group. Asa result, the distance between the first lens group and the second lensgroup can be reduced in a nonuse state to be housed in a thinned style;thus, the imaging lens is suitable for downsizing a camera.

Such a configuration makes it possible to lower the difficulty of theaberration correction, reduce the number of lenses, cut the asphericsurface and reduce the manufacturing error sensitivity compared to acase in which the distance from the most object-side face of the lenssystem to the image face is designed to be small and the distance fromthe most object-side face to the most image-side face of the lens systemis designed to be small.

According to the imaging lens of this embodiment, the configuration ofeach portion is optimized relative to the purpose to generate neweffects, so that a wide angle, large diameter, compactness, low cost andhigh performance can be all acquired.

The condition (1) is to control the entire length of the imaging lens,and the effect can be well obtained within the condition range.

The condition (2) is to control the entire length of the imaging lens,and the effect can be well obtained within the condition range.

By satisfying the condition (3), a high performance and compactness canbe well balanced.

If the parameter exceeds the lower limit of the condition (3), thesecond lens group becomes too close to the first lens group. For thisreason it becomes difficult to sufficiently control the field curvaturebecause the operation as the field flattener by the second lens group isdeteriorated.

If it is considered that the distance between the first lens group andthe second lens group is reduced in a nonuse condition to be housed in athinned style, the effect can not be fully obtained.

If the parameter exceeds the upper limit of the condition (3), the firstlens group can not have a sufficient thickness as the imaging group, sothat it becomes difficult to preferably correct various aberrations.

The condition (4) is to optimize the refractive power of the second lensgroup for downsizing the imaging lens. If the parameter exceeds thelower limit, the refractive power of the second lens group becomes toosmall. For this reason the property as the telephoto type isdeteriorated, and it becomes difficult to reduce the entire length ofthe lens. The operation as the field flattener by the second lens groupis also deteriorated, and the field curvature is not well corrected.

If the parameter exceeds the upper limit of the condition (4), therefractive power of the second lens group becomes too big, so that thefield curvature is excessively corrected, and the coma aberration isgenerated.

The condition (5) is a condition for preferably correcting comaaberration. If the parameter exceeds the lower limit, the inward comaaberration is easily generated while if the parameter exceeds the upperlimit, the outward coma aberration is easily generated. When not onlythe aperture stop but also a shutter is provided between the second andthird lens group, if the parameter exceeds the upper limit of thecondition (5), it becomes difficult to ensure the space for the shutterunit.

The condition (6) is an effective condition for optimizing therefractive power balance in the first lens group and effectively movingthe exit pupil position away from the image face. If the parameterexceeds the lower limit, the refractive power on the object side fromthe apertures stop in the first lens group is excessively increased andthe refractive power on the image side from the aperture stop isexcessively decreased, so that it may not ensure a sufficient exit pupildistance.

If the parameter exceeds the upper limit of the condition (6), therefractive power on the image side from the aperture stop in the firstlens group is excessively increased and the refractive power on theobject side from the aperture stop is excessively decreased, so that thenegative distortion is likely to be generated. The entire balance iseasily deteriorated outside the range of the condition (6) regardinganother aberration.

In order to more preferably correct the coma aberration and astigmatism,it is preferable to adapt on the image-side face of the fifth lens anaspheric surface in which the positive refractive power weakens in theperipheral portion. In this case by satisfying the condition (7), thepreferable correction effect of the coma aberration can be obtained.

If the parameter exceeds the lower limit of the condition (7), theaspheric surface becomes too close to the aperture stop, so that itbecomes difficult to obtain the correction effect of the astigmatismbecause the light beam of the central portion and the light beam of theperipheral portion are not well separated. If the parameter exceeds theupper limit, the aspheric surface stays away from the aperture stop. Forthis reason it becomes difficult to obtain the correction effect of thecoma aberration because the light beam does not have a sufficientdiameter.

The focusing on the close-range object is reliably achieved by movingthe entire lens system. The size of the lens barrel frame to be moved islikely to be increased because the first and second lens groups arearranged via a relatively large interval. The outer diameter of thesecond lens group becomes larger than the outer diameter of the firstlens group because the aperture stop is arranged in the first lensgroup. If a mechanism for integrally moving these is provided, anincrease in a size of the mechanism in the diameter direction isinevitable.

The lens barrel frame to be moved is reduced in length in the opticalaxis direction and in diameter if the focusing on a close-range objectis conducted only by the movement of the first lens group. Accordingly,the moving mechanism can be downsized.

If a mechanism which reduces the interval between the first and secondlens groups for housing in a thinned style is provided, it can becommonalized with the moving mechanism.

As described above, the exchange of large aberration beyond necessity isnot performed between the first lens group and the second lens group, sothat the aberration change is small if the first lens group is onlymoved; thus, a sufficient imaging performance can be maintained.

It is preferable for the three positive lens (second, fourth and fifthlenses) of the imaging lens to satisfy the following conditions (8),(9).

1.75<n_(dpa)<1.95   (8)

35.0<ν_(dpa)<50.0   (9)

Here, n_(dpa) is an average value of the refractive indexes of the threepositive lenses, and ν_(dpa) is an average value of Abbe's number of thethree positive lenses.

By constituting the three positive lenses for imaging with a highrefractive index and low dispersion material which satisfies both of theconditions (8), (9), both of the field curvature and the chromaticaberration can be reduced at high level.

It is preferable for the third and fourth lenses to be cemented to eachother. In order to reduce the final aberration amount, a large exchangeof the aberration is conducted in each face of the third and fourthlenses. Due to the large exchange, the manufacturing error sensitivitymay be increased.

By cementing these lenses, the actual manufacturing error sensitivity isreduced, and a stable performance can be obtained. The number ofcomponents of the lens barrel for actually holding the lenses can bealso reduced.

In order to preferably correct the aberration, it is preferable toprovide an aspheric surface on the first lens. This is effective forcorrecting the spherical aberration and the coma aberration which areincreased due to an increase in an aperture.

As described above, according to the embodiment of the presentinvention, a new imaging lens can be provided. When each of theabove-conditions is satisfied, a small and high-performance imaging lenssuitable for a digital camera with a wide angle in which a half-fieldangle is about 28-36°, a large aperture in which F-number is less than2.8, a small number of lenses and a resolution corresponding to animaging element having 10-20 million pixels can be provided.

Thus, a compact and high-performance camera and personal digitalassistant can be offered by using the above imaging lens.

Hereinafter, embodiments will be described.

FIGS. 1-5 illustrate five embodiments of an imaging lens. Theseembodiments correspond to the after-described Embodiments 1-5. In FIGS.1-5 common reference numbers are used for simplifying the figures, and Idenotes a first lens group and II denotes a second lens group. L1-L6denote first to sixth lenses, respectively, and S denotes an aperturestop. F denotes various filters and a cover glass of an imaging elementas one transparent parallel plate which is optically equivalent tothose.

As illustrated in FIGS. 1-5, each of the imaging lenses of theseembodiments includes in order from the object side (left side in figure)the first lens group I having a positive refractive power and the secondlens group II having a negative refractive power. The first and secondlens groups include therebetween the widest air space in the lenssystem.

The first lens group I includes in order from the object side a negativefirst lens L1 having a concave face on the object side, a positivesecond lens L2 having a convex face on both sides, an aperture stop S, anegative third lens L3 having a concave face on the object side, apositive fourth lens L4 having a convex face on the image side and apositive fifth lens L5 having a convex face on the image side.

The second lens group II includes a negative meniscus sixth lens L6having a concave face on the object side. The third and fourth lensesL3, L4 are cemented in any of the embodiments illustrated in FIGS. 1-5.

The image-side face of the fifth lens L5 is an aspheric surface shape inwhich a positive refractive power weakens in the peripheral portion.

In any of the embodiments in FIGS. 1-5, the first lens L1 is a biconcavelens and the second lens L2 is a biconvex lens. The third lens L3 is abiconcave lens having a large absolute value of curvature on theobject-side face. In the embodiment illustrated in FIG. 5 the third lensL3 is a negative meniscus lens having a large absolute value ofcurvature on the object-side face.

The fourth lens L4 is a biconvex lens having a large absolute value ofcurvature on the image-side face in the embodiments illustrated in FIGS.1-4. The fourth lens L4 includes a positive meniscus lens having a largeabsolute value of curvature on the image-side face.

The fifth lens L5 is a positive meniscus lens having a large absolutevalue of curvature on the image-side face in the embodiments illustratedin FIGS. 1, 2, 4. The fifth lens L5 is a biconvex lens having a largeabsolute value of curvature on the image-side face in the embodimentsillustrated in FIGS. 3, 5.

As illustrated in Embodiments 1-5, the imaging lens illustrated in eachof FIGS. 1-5 satisfies the conditions (1)-(7), and the material of thethird to fifth lenses L3-L5 satisfies the conditions (8), (9).

The embodiment of the personal digital assistant is described withreference to FIGS. 16, 17.

The personal digital assistant includes an imaging lens 31, alight-receiving element (area sensor) 45 as illustrated in FIG. 17, andis configured to read an image of a photographing object formed by theimaging lens 31 with the light-receiving element 45. The imaging lensdescribed above can be used as the imaging lens 31, but the imaging lensdescribed in any of Embodiments 1-5 can be specifically used.

A relatively large imaging element having a diagonal length of about20-45 mm which allows a certain level of oblique incidence of theperipheral light beam onto the imaging face is used.

The output from the light-receiving element 45 is processed in an imageprocessor 41 which is controlled by a central processing unit 40 to beconverted into digital information. The digitized image information issubjected to a predetermined image process in the image processor 41which is controlled by the central processing unit 40, and then isrecorded in a semiconductor memory 44.

An image in photographing can be displayed on a liquid crystal monitor38, and an image recorded in the semiconductor memory 44 can be alsodisplayed on the liquid crystal monitor 38.

An image recorded in the semiconductor memory 44 can be sent outsidewith a communication card 43 or the like.

The imaging lens 31 is in a collapsed state as illustrated in FIG. 16A,and a lens barrel is extended as illustrated in FIG. 16B if a userpresses a power source switch 36 to turn on the device.

Focusing is made upon half-pressing a shutter button 35. In addition, azoom lever is provided, and a so-called digital zooming operation whichartificially changes a magnification according to a change in a cut areaof an image can be performed in response to the operation of the zoomlever. Here, a magnification of a finder 33 is changed together with achange in a field angle.

The focusing can be performed with the movement of the entire lenssystem or can be performed with the movement of the light-receivingelement 45. The focusing can be also performed with the movement of thefirst lens group only as the imaging lens described above.

Upon further pressing the shutter button 35, the photographing isperformed, and then the above process is conducted. The image recordedin the semiconductor memory 44 is displayed on the liquid crystalmonitor 38, and the operation button 37 is used when sending the imageoutside with a communication card 43 or the like. The semiconductormemory 44, the communication card 43 or the like is inserted intodedicated or generalized sockets 39A, 39B. A device except for thecommunication operation with outside is a camera, and this camera is acamera portion of the personal digital assistant.

It is not necessary for each lens group to be arranged on the opticalaxis while the imaging lens 31 is in the collapsed state. The thicknessof the device can be further reduced if the third to fifth lenses L3-L5or the second lens group are/is retracted from the optical axis to behoused in parallel with other lenses.

As described above, the use of the imaging lens of Embodiments 1-5 inthe personal digital assistant makes possible a high image quality andcompact camera and personal digital assistant using a light-receivingelement having 10-20 million pixels (a relatively large imaging elementhaving a diagonal length of about 20-45 mm, which allows a certain levelof oblique incidence of a peripheral light beam onto the imaging face).

Embodiment

Hereinafter, five specific embodiments of the imaging lens will bedescribed.

In Embodiments 1-4 the maximum image height is 14.2 mm and in Embodiment5 the maximum image height is 13.5 mm. In Embodiments 1-5 the parallelplate arranged on the image face side of the second lens group isvarious filters such as an optical low pass filter or an infrared cutfilter, or a cover glass (sealing glass) of the light-receiving elementsuch as a CMOS sensor.

In addition, the parallel plate is arranged such that its image sideface is located on the object side from the imaging face by about 0.5mm, but it is not limited thereto, and it can be divided into aplurality of plates.

The meanings of signs in each Embodiment are as follows.

-   f: focal length of entire optical system-   Fno: F-number-   ω: half-field angle-   R: curvature radius-   D: surface interval-   N_(d): refractive index-   ν_(d): Abbe's number-   P_(g,F): partial dispersion ratio    P_(g, F)=(n_(g)−n_(F))/(n_(F)−n_(c))-   K: conical constant of aspheric surface-   A₄: 4^(th) aspheric coefficient-   A₆: 6^(th) aspheric coefficient-   A₈: 8^(th) aspheric coefficient-   A₁₀: 10^(th) aspheric coefficient

An aspheric surface X is defined by the following known equation with aninverse C (paraxial curvature) of paraxial curvature radius, a height Hfrom the optical axis, the above conical constants and asphericcoefficients.

X=CH ²/[1+√{square root over ( )}(1−(1+K)C ² H ²)]+A ₄ +H ⁴ +A ₆ ·H ⁶ +A₈ ·H ⁸ +A ₁₀ ·H ¹⁰

In each embodiment HOYA in the glass type means HOYA Corporation, OHARAin the glass type means OHARA Corporation, and HIKARI in the glass typemeans HIKARI Glass Corporation.

[Embodiment 1] f = 22.99, F = 2.55, ω = 32.4 SURFACE NUMBER R D N_(d)ν_(d) P_(g, F) GLASS TYPE  01* −19.075 1.00 1.68893 31.08 0.5986 OHARAL-TIM28 02 47.506 0.10 03 18.926 2.16 1.88300 40.76 0.5667 OHARA S-LAH5804 −23.061 0.80 05 APERTURE 4.79 STOP 06 −8.544 0.82 1.69895 30.130.6030 OHARA S-TIM35 07 28.214 2.56 1.83481 42.71 0.5648 OHARA S-LAH55V08 −20.938 0.10 09 −138.070 2.76 1.74320 49.29 0.5529 OHARA L-LAM60  10*−12.915 8.72 11 −11.296 1.20 1.51742 52.43 0.5564 OHARA S-NSL36 12−22.532 VARIABLE (A) 13 ∞ 1.30 1.51680 64.20 VARIOUS FILTER 14 ∞[Aspheric Surface] In the above table a surface number with * (asterisk)denotes an aspheric surface. This is the same in Embodiments 2-5. Theaspheric surface data of Embodiment 1 is as follows. First Surface K =0.0, A₄ = −5.83484 × 10⁻⁵, A₆ = −2.17156 × 10⁻⁷, A₈ = 5.68288 × 10⁻⁹,A₁₀ = −1.59389 × 10⁻¹⁰ Tenth Surface K = 0.0, A₄ = 5.68159 × 10⁻⁵, A₆ =2.68561 × 10⁻⁷, A₈ = −3.11147 × 10⁻¹⁰, A₁₀ = 2.28609 × 10⁻¹¹ [VariableInterval] The variable interval A is an interval change between thefirst and second lens groups due the displacement of the first lensgroup in focusing from an infinite object to a close-range object(magnification of −1/20). Infinity −1/20 times A 7.379 8.528 [Value ofParameter of Condition] [1] L/f = 1.49 [2] Y′/f = 0.617 [3] D_(1G−2G)/L= 0.255 [4] f_(2G)/f = −1.98 [5] R_(L3F)/D_(L2−L3) = −1.53 [6]f_(L1−L2)/f = 1.20 [7] L_(S−A)/L_(S−I) = 0.366 [8] n_(dpa) = 1.820 [9]ν_(dpa) = 44.3

[Embodiment 2] f = 22.99, F = 2.56, ω = 32.5 SURFACE NUMBER R D N_(d)ν_(d) P_(g, F) GLASS TYPE  01* −19.042 1.00 1.68893 31.16 0.6037 HOYAM-FD80 02 32.587 0.10 03 17.848 2.33 1.88300 40.80 0.5654 HOYA TAFD30 04−21.421 0.80 05 APERTURE 4.39 STOP 06 −8.787 0.80 1.68893 31.16 0.5989HOYA E-FD8 07 24.308 3.01 1.83481 42.72 0.5653 HOYA TAFD5F 08 −19.0790.10 09 −59.475 2.36 1.77387 47.25 0.5557 HIKARI Q-LASFH11  10* −14.339VARIABLE (A) 11 −13.524 1.20 1.67270 32.17 0.5962 HOYA E-FD5 12 −22.904 5.619 13 ∞ 1.30 1.51680 64.20 VARIOUS FILTER 14 ∞ [Aspheric Surface]The aspheric surface data of Embodiment 2 is as follows. First Surface K= 0.0, A₄ = −6.48729 × 10⁻⁵, A₆ = −3.64206 × 10⁻⁷, A₈ = 9.41909 × 10⁻⁹,A₁₀ = −2.27481 × 10⁻¹⁰ Tenth Surface K = 0.0, A₄ = 5.05271 × 10⁻⁵, A₆ =3.24752 × 10⁻⁷, A₈ = −1.48743 × 10⁻⁰⁹, A₁₀ = 3.86782 × 10⁻¹¹ [VariableInterval] Infinity −1/20 times A 11.290 12.129 [Numerical Value ofParameter of Condition] [1] L/f = 1.51 [2] Y′/f = 0.617 [3] D_(1G−2G)/L= 0.324 [4] f_(2G) /f = −2.25 [5] R_(L3F)/D_(L2−L3) = −1.69 [6]f_(L1−L2)/f = 1.22 [7] L_(S−A)/L_(S−I) = 0.349 [8] n_(dpa) = 1.831 [9]ν_(dpa) = 43.4

[Embodiment 3] f = 22.99, F = 2.55, ω = 32.7 SURFACE NUMBER R D N_(d)ν_(d) P_(g, F) GLASS TYPE 01 −23.416 1.00 1.54814 45.78 0.5686 OHARAS-TIL1 02 128.083 0.10 03 23.801 1.74 1.88300 40.76 0.5667 OHARA S-LAH5804 −44.679 0.80 05 APERTURE 3.81 STOP 06 −9.949 0.80 1.80518 25.420.6161 OHARA S-TIH6 07 26.658 2.78 1.88300 40.76 0.5667 OHARA S-LAH58 08−24.229 0.10 09 270.532 3.23 1.85400 40.39 0.5677 OHARA L-LAH85  10*−15.271 VARIABLE (A) 11 −11.494 1.20 1.48749 70.24 0.5300 OHARA S-FSL512 −24.356  6.474 13 ∞ 1.30 1.51680 64.20 VARIOUS FILTER 14 ∞ [AsphericSurface] The aspheric surface data of Embodiment 3 is as follows. [TenthSurface] K = 0.0, A₄ = 4.68271 × 10⁻⁵, A₆ = −7.49722 × 10⁻⁸, A₈ =3.11817 × 10⁻⁰⁹, A₁₀ = −1.79903 × 10⁻¹¹ [Variable Interval] Infinity−1/20 times A 10.990 11.783 [Value of Parameter of Condition] [1] L/f =1.51 [2] Y′/f = 0.618 [3] D_(1G−2G)/L = 0.316 [4] f_(2G)/f = −2.00 [5]R_(L3F)/D_(L2−L3) = −2.16 [6] f_(L1−L2)/f = 1.45 [7] L_(S−A)/L_(S−I) =0.344 [8] n_(dpa) = 1.873 [9] ν_(dpa) = 40.6

[Embodiment 4] f = 26.10, F = 2.55, ω = 28.4 SURFACE NUMBER R D N_(d)ν_(d) P_(g,F) GLASS TYPE  01* −23.106 1.00 1.68893 31.16 0.6037 HOYAM-FD80 02 38.954 0.10 03 19.400 2.49 1.88300 40.80 0.5654 HOYA TAFD30 04−25.409 0.80 05 APERTURE 4.02 STOP 06 −10.106 0.80 1.69895 30.05 0.6028HOYA E-FD15 07 40.833 3.15 1.83481 42.72 0.5653 HOYA TAFD5F 08 −14.4270.10 09 −30.482 2.00 1.74330 49.33 0.5527 HOYA M-NBF1  10* −18.907VARIABLE (A) 11 −13.992 1.20 1.58144 40.89 0.5767 HOYA E-FL5 12 −24.992 4.603 13 ∞ 1.30 1.51680 64.20 VARIOUS FILTER 14 ∞ [Aspheric Surface]The aspheric surface data of Embodiment 4 is as follows. First Surface K= 0.0, A₄ = −5.18356 × 10⁻⁵, A₆ = −7.05306 × 10⁻⁸ Tenth Surface K = 0.0,A₄ = 3.82341 × 10⁻⁵, A₆ = 2.11800 × 10⁻⁷, A₈ = −1.03464 × 10⁻⁰⁹, A₁₀ =3.68428 × 10⁻¹¹ [Variable Interval] Infinity −1/20 times A 14.760 15.771[Value of Parameter of Condition] [1] L/f = 1.41 [2] Y′/f = 0.544 [3]D_(1G−2G)/L = 0.401 [4] f_(2G)/f = −2.18 [5] R_(L3F)/D_(L2−L3) = −2.10[6] f_(L1−L2)/f = 1.12 [7] L_(S−A)/L_(S−I) = 0.310 [8] n_(dpa) = 1.820[9] ν_(dpa) = 44.3

[Embodiment 5] f = 20.01, F = 2.43, ω = 35.3 SURFACE NUMBER R D N_(d)ν_(d) P_(g,F) GLASS TYPE  01* −19.041 1.00 1.68893 31.16 0.6037 HOYAM-F080 02 35.375 0.10 03 18.449 1.99 1.88300 40.80 0.5654 HOYA TAFD30 04−21.673 0.80 05 APERTURE 6.17 STOP 06 −7.627 1.08 1.84666 23.78 0.6191HOYA FDS90 07 −40.431 2.88 1.83481 42.72 0.5653 HOYA TAFD5F 08 −12.6030.10 09 84.682 4.43 1.80610 40.73 0.5693 HOYA M-NBFD130  10* −15.269VARIABLE (A) 11 −19.987 1.20 1.84666 23.78 0.6191 HOYA FDS90 12 −104.3984.672 13 ∞ 1.30 1.51680 64.20 VARIOUS FILTER 14 ∞ [Aspheric Surface] Theaspheric surface data of Embodiment 5 is as follows. First Surface K =0.0, A₄ = −6.73538 × 10⁻⁵, A₆ = −3.07899 × 10⁻⁷, A₈ = −1.81784 × 10⁻⁹,A₁₀ = 6.98371 × 10⁻¹¹ Tenth Surface K = 0.0, A₄ = 5.50839 × 10⁻⁵, A₆ =8.64191 × 10⁻⁸, A₈ = 3.64358 × 10⁻¹⁰, A₁₀ = 6.27272 × 10⁻¹³ [VariableInterval] Infinity −1/20 times A 8.580 9.238 [Value of Parameter ofCondition] [1] L/f = 1.74 [2] Y′/f = 0.675 [3] D_(1G−2G)/L = 0.247 [4]f_(2G)/f = −1.46 [5] R_(L3F)/D_(L2−L3) = −1.09 [6] f_(L1−L2)/f = 1.42[7] L_(S−A)/L_(S−I) = 0.474 [8] n_(dpa) = 1.841 [9] ν_(dpa) = 41.4

FIG. 6 provides aberration curves when the imaging lens in Embodiment 1is focused on an infinite object. FIG. 7 provides aberration curves whenthe imaging lens in Embodiment 1 is focused on a close-range object by−/20 times.

FIG. 8 provides aberration curves when the imaging lens of Embodiment 2is focused on an infinite object. FIG. 9 provides aberration curves whenthe imaging lens of Embodiment 2 is focused on a close-range object by −1/20 times.

FIG. 10 provides aberration curves when the imaging lens of Embodiment 3is focused on an infinite object. FIG. 11 provides aberration curveswhen the imaging lens of Embodiment 3 is focused on a close-range objectby − 1/20 times.

FIG. 12 provides aberration curves when the imaging lens of Embodiment 4is focused on an infinite object. FIG. 13 provides aberration curveswhen the imaging lens of Embodiment 4 is focused on a close-range objectby − 1/20 times.

FIG. 14 provides aberration curves when the imaging lens of Embodiment 5is focused on an infinite object. FIG. 15 provides aberration curveswhen the imaging lens of Embodiment 5 is focused on a close-range objectby − 1/20 times.

In each aberration curve the dashed line in the spherical aberrationillustrates a sine condition, and the solid line and the dashed line inthe astigmatism illustrate sagittal and meridional, respectively.

The aberration of each embodiment is corrected at a high level, and thespherical aberration and the axial chromatic aberration are very small.The astigmatism, the field curvature and the chromatic aberration ofmagnification are small enough, the coma aberration and the colordifference are effectively controlled, and the distortion is 4.0% orbelow at an absolute value.

As described above, it is apparent from the above embodiments that theimaging lens includes a wide angle having a half-field angle of about28-36°, a large aperture having an F-number of less than 2.8 whileensuring a preferable image performance.

Although the embodiments of the present invention have been describedabove, the present invention is not limited thereto. It should beappreciated that variations may be made in the embodiments described bypersons skilled in the art without departing from the scope of thepresent invention.

1. An imaging lens, comprising: in order from an object side a firstlens group including a positive refractive power; and a second lensgroup having a negative refractive power, the first lens group and thesecond lens group including a widest air space therebetween, the firstlens group including in order from the object side a negative first lenshaving a concave face on the object side, a positive second lens havinga convex face on both sides, an aperture stop, a negative third lenshaving a concave face on the object side, a positive fourth lens havinga convex face on an image side and a positive fifth lens having a convexface on the image side, and the second lens group including a negativemeniscus sixth lens having a concave face on the object side.
 2. Theimaging lens according to claim 1, wherein a distance L from a mostobject-side face of the first lens group to an image face in a statefocused on an infinite object and a focal length f of an entire systemin a state focused on an infinite object satisfy the following condition(1).1.2<L/f<2.0   (1)
 3. The imaging lens according to claim 2, wherein thefocal length f of the entire system in a state focused on an infiniteobject and a maximum image height Y′ satisfy the following condition(2).0.50<Y′/f<0.70   (2)
 4. The imaging lens according to claim 1, wherein adistance L from a most object-side face of the first lens group to animage face in a state focused on an infinite object and a distanceD_(1G-2G) from a most image-side face of the first lens group to a mostobject-side face of the second lens group in a state focused on aninfinite object satisfy the following condition (3).0.15<D _(1G-2G) /L<0.50   (3)
 5. The imaging lens according to claim 1,wherein a focal length of the second lens group f_(2G) and a focallength f of an entire system in a state focused on an infinite objectsatisfy the following condition (4).−2.7<f _(2G) /f<−1.2   (4)
 6. The imaging lens according to claim 1,wherein a curvature radius R_(L3F) of an object-side face of the thirdlens and a distance D_(L2-L3) from an image-side face of the second lensto an object-side face of the third lens satisfy the following condition(5).−2.5<R _(L3F) /D _(L2-L3)<−1.0   (5)
 7. The imaging lens according toclaim 1, wherein a synthetic focal length f_(L1-L2) of the first andsecond lenses and a focal length f of an entire system in a statefocused on an infinite object to satisfy the following condition (6).1.0<f _(L1-L2) /f<1.8   (6)
 8. The imaging lens according to claim 1,wherein the fifth lens includes as an image-side face an aspheric shapein which a positive refractive power weakens in its peripheral portion.9. The imaging lens according to claim 8, wherein a distance L_(s-A)from the aperture stop to the aspheric surface of the fifth lens and adistance L_(s-I) from the aperture stop to the image face satisfy thefollowing condition (7).0.25<L _(S-A) /L _(S-I)<0.55   (7)
 10. The imaging lens according toclaim 1, wherein the first lens is only moved when focusing on aclose-range object.
 11. A camera comprising the imaging lens accordingto claim 1 as a photographing optical system.
 12. A personal digitalassistant comprising the imaging lens according to claim 1 as aphotographing optical system of a camera function portion.